The darkest corner of this search involves databases of leaked passwords from unrelated breaches. The idea is to use credential stuffing (trying the same email/password from a LinkedIn or Adobe hack) against ThisVid accounts with private videos.
: Inputting your login credentials into a third-party tool claiming to be a viewer allows scammers to hijack your account.
If the video is old, the uploader may be gone. But ThisVid does delete accounts quickly. Some users have had success searching for the uploader's username on other platforms (Reddit, Twitter, FetLife) and sending a message there. This is tedious, but it is 100% legal.